Circuits referred to as converters are conventionally used in many control apparatuses. The converter has a current-limiting circuit composed of a current-limiting resistor, a rectification circuit composed of a diode, and a smoothing circuit composed of a reactance and an electrolytic capacitor. The converter has a function for converting AC current output to DC current. The electrolytic capacitor disposed in the converter is a crucial component for supplying stable DC voltage to an inverter. There is a high probability of damaging the current-limiting resistor and a power transistor of the inverter in terms of a secondary failure if a short-circuit fault occurs in an electrolytic capacitor and in a load that is connected in parallel to the electrolytic capacitor. In order to prevent this type of failure, control apparatuses are widely used that detect a voltage between the terminals of an electrolytic capacitor and diagnose a failure (e.g., see Japanese Laid-open Patent Publication No. 3-22821).